Evolution of the TransportNetwork

Deirdre Doherty, Thomas Müller, Newman Wilson

W H I T E P A P E ROPTICAL NETWORKING GROUP

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INTRODUCTION

In today's telecommunications environment,change is rapid and pervasive: traffic is growingexponentially, fuelled by both consumer andbusiness demands; new optical technologies areallowing capacity to keep pace with demand,and are enabling new services; the regulatoryenvironment is changing almost daily; andcompetition is intensifying on all fronts. Insuch an environment, choosing the righttransport network, and the proper evolutionarypath toward realising it, becomes a difficult andperilous proposition, as there are manyvariables to weigh, and substantial risks toconsider. In this paper, we discuss the key factorsaffecting the evolution of the transport network,and provide guidance for future-ready thetransport infrastructure.We begin by viewing traffic growth as the keydriver behind the evolution of the transportnetwork, and optical technology as the keyenabler of transport evolution. We then presentthree transport architecture alternatives andconsider the merits of each relative to futureextension.Finally, we describe the needs of the transportnetwork, and the elements of Lucent's transportnetwork vision and evolution. We show that thebenefits accruing to service-provider customersinclude flexible capacity management and atransport infrastructure that is truly future-ready.

TRAFFIC DEMAND GROWTH

Figure 1 shows the projected growth in data-trafficdemand over a four-year period beginning with theyear 1998. Note that by the year 2002, growth intraffic demand is projected to increase by a factorof 16. This exponential growth assumes, of course,that the applications fuelling demand (voice, data,web, e-mail, file transfer, video, videoconferencing, multimedia) will change and evolveover the next several years, as will the networkprotocols that must efficiently support them.

The other half of the demand equation is, ofcourse, a transport network of sufficient capacityand flexibility to accommodate exponential growthand near-limitless change. Transport infrastructuremust evolve, therefore, in such a way as to providereliable, flexible, manageable networks that inter-operate among themselves, and that support long-term traffic growth independent of particularapplications or protocols.

TRANSPORT CAPACITY GROWTH

The rate of growth in optical-transport capacity isexpected to exceed the growth rate in data-trafficdemand over the next several years, and also tosurpass the rate of growth in switch/routercapability (see Figure 2). The following scenariossupport this outlook.

We can extrapolate the actual growth rates inoptical transport by taking the commercial releaseof the first 2.5 Gbps systems in 1991, and the first400 Gbps systems in 1998, as the basis for theextrapolation. Doing this yields a growth rate ofabout 73% per year. We can then compare thisgrowth rate to Moore's Law, which governs thegrowth of Routers and Switches. Moore's Law

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states that the number of transistors on a chipdoubles every 18 months, thus yielding a growthrate of 46% per year.

Other studies estimate the optical growth rate at130% per year through Year 2001, and estimatethe switch/router capacity growth rate at 20 to50% per year1. A Ryan Hankin and Kent (RHK)report on WDM2 forecasts optical-capacity growthexceeding the switch/router capability growth. Thisreport also shows the reduction in the cost ofWDM-based transport.

The extraordinary growth in transport capacitysuggested above will continue to be achieved by acombination of electrical and wavelengthmultiplexing technologies.

TRANSPORT NETWORK EVOLUTION

ARCHITECTURE

Evolution of the optical transport networknecessarily involves evolution of the underlyingarchitecture. There are three primary options forevolving the optical-transport architecture:

• Present Mode Operation

• Transport Layer Networking

• Client Layer Networking

These options are illustrated in Figure 3.

The Present Mode Operation option (CL, SDH,Optical) allows for evolving functionality andfeatures in the three network layers independently.The SDH/SONET layer in this approach follows

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(Reduced SDH/SONET functionality

integrated with client layer)

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SDH/SONET SDH/SONET SDH/SONET

Optical (WDM) Optical (WDM) Optical (WDM)

Figure 3: Transport Network Architecture Options1 VISIBIS WORKSHOP "IP OVER DWDM", JULY 1999, GENEVA, SWITZERLAND. 2 RHK REPORT "WDM AND OPTICAL NETWORKS: TECHNOLOGY AND MARKETS", 1999.

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well-established standards. Conforming to thesestandards helps fulfil the interoperability needs ofembedded-base equipment, and theinterconnection granularity (E1, E3, etc.) needs fora large fraction of leased-line capacity.

In the Transport Layer Networking option, thelower two network layers are integrated, whilekeeping standards and interoperabilityrequirements intact. The client-layer equipment(switches/routers) implements theSDH/WDM/optical interfaces functionality (toconnect to the transport-layer network), but doesnot provide transport-layer networkingfunctionality. By integrating functions in commonequipment, there is potential in the near term forboth equipment and operations cost savings. In thelong term, functionality to can evolve in a unifiedinfrastructure.

Note that the investment at the physical layer(involving buried cables and associated equipment)tends to have a much longer life, during whichthere can be many life-cycle changes in the upper-layer protocols, the applications, and thecorresponding upper-layer equipment. In theunified physical infrastructure, an integratedmanagement approach is possible, which affordsmany key benefits, including provisioningflexibility; improved reliability, availability, andnetwork efficiency (stemming from an integrated

approach to fault detection); protection switching;and restoration.

In the Client Layer Networking option, theupper-layer equipment (routers and switches)incorporates part of the functionality provided atthe SDH/SONET layer, including protectionswitching in dual-ring architectures. Using thisapproach alone limits service-provider networks toring implementations, but with cost reductionscomparable to those in Transport Layer Networkingequipment integrations, as the twoimplementations are very similar. The reducedfunctionality, however, can limit fullinteroperability as well as the flexibility associatedwith complete standards-based equipment.

While it is possible to incorporate additionalfeatures to integrate protection functions withother features (associated with Upper LayerProtocol), such implementations tend to beproprietary, and do not lend themselves tooperations in a multi-vendor, multi-serviceenvironment. In addition, there are concerns aboutthis solution's scalability to large service-providernetworks. As mentioned previously, the life-cycleof the client-layer (router) equipment is shorter,and the SDH functionality in the combinedequipment may face premature retirement. Inaddition, there are additional costs associated withequipment swap-out operations.

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Present Mode Transport Layer Client Layer Operation Networking Networking

Current Follows well- Aimed at retention and Reduced functionalitystandards- established SDH extension of SDH- (line-card typebased standards for full analogous features in implementation) &functionality flexibility & inter- Optical/WDM restricted inter-

working operabilityStandards Independent evolution Integrated functionality Proprietary extensionsfunctionality of functionality in all in a unified for a ring architectureextension layers infrastructureLife-cycle cost Separate long time- Separate long time- Pre-mature SDH swapcontainment scale infrastructure scale infrastructure out along with

equipment equipment switch/routerOperations/ Flexibility in Potential operations Operations efficiencyupgrades (incremental) capacity efficiency but swap-out upgrade-

allocation related costsStandards Standardised Standardised Proprietary-based inter- switch/router (SDH) switch/router (SDH) implementations innetworking interfaces employed interfaces employed switch/router interfacesScalability for Proven in large Scalable to large Scalability concernslarge networks networks networks exist

Table 1: Transport Architecture Comparison

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The three architecture options outlined above arecompared in Table 1.

LONG-TERM REQUIREMENTS

This section identifies and briefly discusses thelong-term requirements of the optical transportnetwork:

• Open multi-service platform.

• Flexibility.

• Protection and restoration.

• Scalability.

• Management and supervision.

Open Multi-Service PlatformNext Generation Networks will need to provide aplatform for an ever-growing number of differentservice types, today's existing and currently-developing services as well as all future services.These new networks will need to allow both largeand small companies to develop networkapplications and to offer associated services on thepublic network (Programmable Networks). Theconcept of Programmable Networks will leveragethe creativity of new enterprises, and will be one ofthe sources of the anticipated exponential trafficgrowth.

In order to support a large variety of services andapplications, and Programmable Networks, theunderlying infrastructure will need to be highlyflexible, and will require easy adaptations to anever-changing environment. Therefore, theinfrastructure of the Next Generation Networkmust be open, and must support a large variety ofdifferent services.

Exponential traffic growth, together with explodingservice variety, will affect optical-networkarchitectures at the client layer. The protocolsinvolved will likely need to undergo significantchange; the various network elements requiredwill certainly need to be adapted to higher trafficloads; and even the client-layer logical topologiesthemselves will likely need to undergo significantchange. In addition, because advancement inoptical-networking technology is so rapid andradical, and because Programmable Networks willprovide such an extensive variety of newapplications, new client layers based ontechnologies not yet known will need to bedeveloped and implemented.

However, service providers cannot afford tosupplant existing transport infrastructure in orderto accommodate changes at the client layer. Hence,an Open Multi-Service Platform inevitably requiresan optical-transport infrastructure that isindependent of changes at the client layer, and thatprovides a pool of types of shared functionality; inother words, it requires Optical Transport LayerNetworking.

There is also an economical advantage to providingan Open Multi-Service Platform by way of OpticalTransport Layer Networking: A transport layer in anetwork allows different services to share rawbandwidth. Even in the case where bandwidthrequirements of different services might justifyseparate transport infrastructures, the savings inoperations for a common basic transportinfrastructure would make a common transportinfrastructure far superior.

FlexibilityThere will be continued need to manage capacitiesin transport networks. This need is induced by thefact that different services may have different peaktimes, certain capacities may be leased for limitedperiods, and main traffic relations may change overtime. An example for the latter would be tradefairs creating - during different times of the year -temporary traffic peaks in different parts of anetwork (Figure 4). In this example, arearrangement of the existing transport capacitycould increase capacity use and defer adding newcapacity to the network. Also, operations,upgrades, and repairs of portions of the networkwould require temporary rearrangement of trafficand hence some network flexibility.

Key to implementing flexibility into an opticalnetwork is the eventual introduction of opticalcross-connects and flexible add-drop multiplexers,as well as associated network management.

In sum, optical transport networks will eventuallyrequire flexibility similar to today's transportnetworks. In order for optical transport networksto be shared among a variety of client layers andservices, a clear separation must be made betweenthe optical-transport layer and the client layer. Thisapproach increases the overall efficiency of thenetwork.

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Protection and RestorationProtection or restoration mechanisms can beimplemented either in the client network or in thetransport network3.

At first glance, it might seem preferable to leaveprotection to the client layer, because doing thiswould allow protection to be customised arounddifferent services. However, there are moreadvantages to implementing protection/restorationmechanisms in the transport layer:

• Protection bandwidth can be shared betweendifferent applications (for example, betweenOptical Shared Protection Rings and Opticalrestoration), thereby providing more-efficientbandwidth usage (Figure 5).

• Unified protection mechanisms provideoperational advantages; for example, personneland the network management system do notneed to deal with an otherwise excessive varietyof different protection mechanisms

• Leased-capacity services require that links witha specified level of reliability be written down ina service-level agreement. This in turn requiresprotection mechanisms in the optical layer.

ScalabilityScalability is fundamentally important for planningand deploying new technologies. Scalability is ameasure of a network's ability to grow in numberof users, number of network nodes, geographicreach, and total bandwidth. The challenge is toachieve scalability within the confines of othernetwork requirements, especially those pertainingto cost, performance, and reliability.

Consider the network example in Figure 6. Theoperator has deployed a client-layer transportarchitecture, and certain nodes experience suddenspikes in demand; for example, a major newcustomer comes on line. In Figure 6, to handle theincreased capacity, the entire network must beexpanded. This expansion includes addingintermediate client-layer nodes to provide

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Figure 4: Example of Network Flexibility

3 FOR A MORE DETAILED DISCUSSION OF PROTECTION OPTIONS, PLEASE REFER TO THE WHITE PAPER "MULTI-LAYER

SURVIVABILITY" BY JOHAN MEIJEN, EVE VARMA, REN WU, AND YUFEI WANG.

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transport-like bandwidth management andsurvivability functions for the engineered routes.

The problem is that the client-layer logical topologybecomes tied to the network's physical-linktopology. Our analysis shows that this couplingleads the more complex and expensive client layerto become de-optimised. At a time when there isso much growth and churn in new services,network operators cannot afford this kind of de-optimisation

Transport networking provides a better solution, asshown in Figure 7. By implementing networking,bandwidth management, and survivability foraggregated traffic in the most efficient way, theclient layer is freed to grow and operate in themost effective manner possible. Under thisapproach, expensive, client-layer upgrades are

made only when they are absolutely required,while the reliable transmission achieved - providedby transport networking between service endpoints- makes it easier to ensure Quality of Service.

Management and SupervisionIn the context of the evolving transport network,the optical network will need to assume more andmore of the functions currently provided bySDH/SONET networks. This is certainly true ofprotection mechanisms, as we discussed previously,but is perhaps especially true of networkmanagement functions. For example, theparameters of Quality and Service LevelAgreements will need to be monitored andcontrolled, not just at the edges, but also indifferent sections of the network. In addition,multi-vendor environments and high connectivity

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Figure 5: Protection Sharing between Different Applications

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between separate networks will require monitoringof optical channels between different networkislands. Also, leasing wavelengths will requireusage of the entire network-managementfunctionality currently used in SDH/SONETnetworks. This functionality can only be providedby the transport-network architecture. In addition,large transported bandwidths, representing highfinancial values, will need to be protected bysophisticated monitoring and measuringcapabilities.

In order to meet these requirements for networkmanagement, Lucent Technologies is developing adigital-wrapper technology, called WaveWrapper™Technology, that provides optical-channel overheadindependent of input-signal format. This overheadprovides full network management capabilities in a

virtually transparent network environment. SeeFigure 8.

SHORT-TERM REQUIREMENTS

Today, the SDH transport network provides thefeatures of an open, integrated transport platform.The SDH transport infrastructure provides efficientcapacity-sharing between different applications,highly flexible network elements like ADM andDXC, and reliable transport links. Also, SDHstandards allow for extensive networkmanagement capabilities. In short, the great successof SDH networking is due to the fact that itprovides the transport-networking functionalitiesfor today's applications.

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Figure 7: Client Layer Scalability

Figure 8: Optical Channel "Digital Wrapper"

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The high reliability and quality of service of leasedlines provide a still booming market for SDH andSONET equipment. Both incumbent and newoperators - even those operators determined todevelop a completely IP-based infrastructure - offerleased-line services at SDH/SONET bit-rates today.

Eventually, there will be more and moreequipment that will interface to the transport layerat bit-rates matching the maximum SONET/SDHbit-rates. The SONET/SDH network will then movetoward the edges of the transport network, and itsfunctionality in the backbone and regionalnetworks will be absorbed into the opticaltransport network.

In the meantime, the advantages of an establishedSDH transport network and network-managementenvironment should be exploited.

CONCLUSION

By the year 2002, growth in traffic demand isprojected to increase by a factor of 16. Capacitygrowth in the transport layer is expected to besufficient to accommodate this projected trafficgrowth. However, capacity growth in the client(switching/routing) layer is expected to lag farbehind that of the transport layer.

The network protocols needed to efficientlysupport various applications will evolve, and theclient layer networking equipment will changeaccordingly. Hence, the infrastructure of thetransport network must evolve in such a way as toprovide reliable, flexible, manageable networksthat inter-operate among themselves, and thatsupport long-term traffic growth independent ofparticular applications or protocols.

Transport-layer networking can provide a reliable,flexible, and manageable network sufficientlyscalable to accommodate both near-exponentialdemand growth and near-limitless change.Transport-layer networking can supportinteroperability and long-term traffic growthindependent of client applications and networkprotocols. Networking at the transport layerovercomes the serious architecture (and other)limitations of networking at the client layer.

An efficient transport network infrastructurerequires separation of the transport and clientlayers. Transport functionality should not beincluded in the client layers, because suchintegration leads to inefficiencies resulting from alack of sharing between different applications. Italso leads to a lack of scalability and higheroperations costs.

Today, transport-layer networking is provided bySDH/SONET standards-based networks. Astransport networks evolve, transport-layernetworking will migrate into the optical transportnetwork.

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GLOSSARY

Abbreviations Used:

ATM Asynchronous Transfer Mode

CL Client Layers

DXC Digital Cross Connect

DWDM Dense Wavelength Division Multiplexing

FDDI Fiber Distributed Digital Interface

FEC Forward Error Correction

GbE Gigabit Ethernet

Gbps Gigabits per second

IP Internet Protocol

OADM Optical Add/Drop Multiplexer

OAM Operations, Administration, and Maintenance

OCh Optical Channel

OXC Optical Cross Connect

PDH Plesiochronous Digital Hierarchy

SDH Synchronous Digital Hierarchy

SDL Simplified Data Link

SONET Synchronous Optical NETwork

WDM Wavelength Division Multiplexing

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